This invention is directed to an adhesive patch for repair of composite structures, an adhesive for that patch and to a method of forming the adhesive patch. Further, the invention is directed to a method of repairing a damaged composite structure. The adhesive for the adhesive patch is based upon "toughened" bismaleimide chemistry.
Composites are increasingly being used for structural components in aircraft. In a manner similar to their metallic counterparts, composite structural components for aircraft can sustain damage. When so damaged it is necessary to repair the composite structure. In repairing a composite structure an adhesive is utilized to join a sheet of a "prepreg" over the damaged area of the composite structure. "Prepregs" are sheets of uncured resin impregnated into composite structural fibers. Depending on the properties of the uncured resin, a binder may or may not be necessary to hold the prepreg together prior to curing. In order for the prepreg to adhere to the composite structure, an adhesive must be interspaced between the composite structure and the prepreg. After adhering the prepreg to the composite structure, it can then be cured to integrate it with the composite structure.
Composites based upon bismaleimide chemistry have certain inherent advantages over other composite systems such as epoxy composites. These include high elastic modulus, high glass transition temperatures and improved fracture toughness, i.e. impact resistance. This makes such bismaleimide composites particularly useful for aircraft structures.
Bismaleimide resins are generally synthesized from aromatic diamines and maleic acid anhydride. The reaction between the diamine and the maleic acid anhydride, however, liberates certain volatiles. Because of the liberation of these volatiles, if aromatic diamines and maleic acid anhydride are utilized as an adhesive, they can give rise to voids along the bond line during the repair of a composite structure. Such voids obviously impair the integrity of the repaired structure rendering this repair highly undesirable. In order to avoid the production of volatiles, low molecular weight imide prepolymers that already contain an imide functionality can be utilized. Bismaleimide structures contain two such imide functionalities joined by a bivalent linking group, as for instance an aromatic or aliphatic linking group. When used alone, however, such bismaleimide structures tend to be extremely brittle and subject to fracture.
In order to improve the properties of bismaleimides it has been suggested by Shaw, et al., J. Adhesion and Adhesives, 123, July 1985, to "toughen" bismaleimides with carboxyl-terminated butadiene-acrylonitrile. In an extension of Shaw's work, Stenzenberger, et al., 19th International SAMPE Technical Conference, Oct. 13 through 15, 1987, suggested that a bismaleimide/bispropenylphenoxy resin system could be modified to toughen the resin by the addition of the carboxy terminated butadiene-acrylonitrile rubbers of Shaw et al. In forming such a modified resin, these authors melt blended the bismaleimide resin with the rubber component followed by curing.
Chaudhari, et al., SAMPE Journal, July/August 1985, describes a bismaleimide resin system known as XU292. XU292 is commercially available from the Ciba-Geigy Corporation. Its components are 4,4'-bismaleimidodiphenylmethane and o,o'-diallylbisphenol A, i.e. 2,2'-bis(3,3'-diallyl-4,4'-phenoxy)propane. As is shown by Chaudhari, et al., a blend of these two monomers undergoes a prepolymerization at 110.degree. C. to 125.degree. C. At this temperature Chaudhari, et al. found that a clear homogenous melt was formed that underwent a prepolymerization reaction of an "ene" type.
While we do not wish to be bound by theory, it is believed that the system of Shaw et al. or Stenzenberger et al. in forming a melt, would undergo the same prepolymerization as discussed by Chaudhari, et al. Chaudhari, et al further discloses. that the 4,4'-bismaleimidodiphenylmethane and o,o'-diallylbisphenol A resin system undergoes a second reaction at a higher temperature at about 250.degree. C. wherein polymerization between the two monomers is complete.
In U.S. Pat. No. 4,471,728 to Katley a low temperature single component polyamide adhesive is discussed. The shelf life of this adhesive is indicated to be six months at 0.degree. C., 3 weeks at 25.degree. C., but only, as a maximum, 5 days at 40.degree. C. 40.degree. C. is typical of storage or shipping temperatures that would occur in transport and use of such an adhesive. The storage properties of the Katley adhesive are fairly indicative of storage properties of all single system or single component composite adhesives. That is, unless these adhesives are stored at or below 0. they have a very poor shelf life. The only known way to solve this storage problem is to use a two component adhesive and to mix the two components of the adhesive immediately prior to use. Such two component adhesives, however, are very difficult to use in a "field" type setting since for optimum performance the exact quantities of the two components of the adhesive must be accurately weighed out prior to mixing. Further, after mixing, such adhesives must be applied to a scrim cloth in a thin uniform layer. The scrim cloth bearing the adhesive is then applied to the composite structure in need of repair. All of this is difficult if not impossible to do at a field operation level.